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The difference between Linewidth and Bandwidth in laser

odawnw

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Linewidth and Bandwidth - It's very difficult for me to distinguish between these two parameters in laser, at first I think they must be identical, but, after checking the definition, I find that I am confused.

Let's take an example.
Now that I need the laser to produce a beam which has a full width at half maximum (FWHM) within 0.112237 [nm], and the corresponding frequency might be 2.67106620811e [EHz], there is no doubt that this is the bandwidth of of my requirement.
however, according to the specification of many laser systerms, their linewidth might be only within MHz (some might be in GHz, but rare), may I understand that these laser cannot meet me requirement?

I would like to hear your suggestion.
Thanks!
 



RedCowboy

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I know bandwidth is basically how much information it can carry although that's not what you're asking.

A 405nm Blu-Ray has more wave fronts per inch than a 660nm red, thus more digitized information can be carried per inch on a 405nm beam than a 660nm beam as they both move at the same speed, the speed or light, but the lower the number ( NM ) the higher the frequency and the more bandwidth.

405 nm is the actual length of the wave as 660nm is the length of that wave, so the 405 has more waves per given length/duration of time.

This is why South Korean internet is so fast, it's using a lot of optical fiber, while they invested in fiber our infrastructure invested in wireless.

In what context are you using linewidth? That can mean different things too.
 
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odawnw

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Bandwidth is basically how much information it can carry.

A 405nm Blu-Ray has more wave fronts per inch than a 660nm red, thus more digitized information can be carried per inch on a 405nm beam than a 660nm beam as they both move at the same speed, the speed or light, but the lower the number ( NM ) the higher the frequency and the more bandwidth.

405 nm is the actual length of the wave as 660nm is the length of that wave, so the 405 has more waves per given length/duration of time.

This is why South Korean internet is so fast, it's using a lot of optical fiber, while they invested in fiber our infrastructure invested in wireless.

In what context are you using linewidth? That can mean different things.
thanks for your reply
For my above example, I focus on the laser spectrum's phase only, but not any information carried inside.
To make it clear, I would like to elaborate my example as a plot below, let's assume the plot below is the desired spectrum I hope that I can get from the laser I buy with a laser producer

The full width at half maximum (FWHM) in this plot is the black horizontal line, ignoring the axis (actually this is a photo I find in Google image) let's say the line's width is 0.112237 [nm], which is 2.67106620811e [EHz] according to the formula in https://www.rp-photonics.com/bandwidth.html

The question now is that, nearly no laser producer offer the parameter in this aspect, all they offer are only linewidth, but in this case, can a linewitdh equal to the bandwidth?
 
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LSRFAQ

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Start researching using the terms Etalon, ECDL laser and DFB laser
You place a carefully designed temperature tuning system and some diffraction grating feedback around the diode. For the very narrow linewidth your describing, you possibly lock the diode current to an atomic adsorption system to slightly modulate the wavelength. I'm not sure if you can get that narrow with a diode, so start reading. Review of Scientific Instruments and other journals are full of data on this. The low noise diode driver required is an artform in itself. RSI has a few good designs.
Steve
 

LSRFAQ

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Whoops, I just responded to a 2016 Thread.. Dang "suggested" posts...

Steve
 

RedCowboy

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Pretty interesting posting, I had the same question
Can you define the difference now adalah?

Do you understand how two lasers of the same advertised bandwidth can have very a different linewidth ?

Can you tell me the difference between spectral width and linewidth ?
 
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smallfreak

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Can you define the difference now adalah?

Do you understand how two lasers of the same advertised bandwidth can have very a different linewidth ?

Can you tell me the difference between spectral width and linewidth ?
Are you sure there really is a fundamental difference? If „bandwidth“ is defined on the frequency graph instead of a measure of information throughput, then „linewidth“ is associated with spectrum charts. At least that‘s the only place where I have been confronted with this attribute so far. There the „linewidth“ is a measure of the slit projection in the measured spectrum for a given wavelength. A smaller slit defines a clearer line, a broader slit defines a more fuzzy one.

But then again, measuring the intensity of such „lines“ is the way to get the spectral chart and the „line width“ directly corresponds to the „bandwith“ on the other graph. So the bandwidth is a cross section of the line width.

Do you have an example where these properties are handled differently in a meaningful manner? I am not too old to learn something new.
 

RedCowboy

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I have seen the terms used as equal so I am looking for the context in which there is a profound difference myself.

It's kind of like saying it takes a lot of money to build an oceangoing hovercraft and you shouldn't even get started until you have the cash on hand.

Now ( in this context ) is money defined as the amount of currency that someone has in the bank that may not all be liquid and is cash defined as the currency that someone has on hand that is liquid ? Or is money and cash not basically the same thing ?

I am hoping to learn myself.

---edit---
Maybe this will help.


 
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Singlemode Laser

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Laser linewidth and laser bandwidth mean the same but linewidth is used more often to describe a narrow spectrum for a single frequency laser and bandwidth usually a broader spectrum.

Singlemode
 

smallfreak

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Now ( in this context ) is money defined as the amount of currency that someone has in the bank that may not all be liquid and is cash defined as the currency that someone has on hand that is liquid ? Or is money and cash not basically the same thing ?
Well, as far as I understand that, "money" ist still "easily available finanical ressources of various kind" as opposed to "capital", which includes bound values. "Cash" is the explicit physical expression of money, usually in form of bank notes and coins, so an even more immediate availability to "virtual money" that only consits of numbers in a database where the holder of this database assures you theat it is equivalent to real value you can spend.

Technically it's the difference in who is offering the warranty of value. As long as the "money" is hold by your bank account as virtual checkbook money, it's the bank that warrants the value - or maybe not, if we look at the "Lehman Brothers". "Cash" makes use of the governmental issued debt obligations i.e. paper money and coins.

And we should not forget the fact that 90+% of all "money" is just hot air. A promise of a private company (bank) that you might buy goods with these numbers. This promise is backed by nothing other than the public believe that it is true. As soon as more than 10% of the customers question this promise and insist to rather get a governmental granted value (i.e. cash), the systsem implodes, as the bank simply cannot convert more than 10% (or even lower) of their virtual money into "real money".

And while we are at it - the governmental "promise" is worth almost nothing too as these debt obligations are based on hot air themselves since president Nixon abandoned the gold standard in 1971 - and the world with him.
:rolleyes:
 




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